Gamma delta T lymphocytes represent a minor subpopulation of T cells which are found only in small numbers in peripheral lymphoid organs, but which represent the predominant T cells in many epithelial sites (skin, tongue, gut, vagina). Although the overall structure of the gamma delat T cell receptor (TCR) appears to resemble that of the alpha beta TCR, very little is known about the nature of the antigen and the presentation molecules (restriction elements) recognized by the gamma delta TCR. Major goals of our studies are to characterize the ligand for the gamma delta TCR and to define the role of this T cell subpopulation in normal immunoregulatory processes. We have designed chimeric genes encoding the VDJC regions of the gamma- and delta- chains fused to the hinge, CH2 and CH3 domains of human IgG1. Transfection of COS cells with these constructs resulted in the secretion of a soluble gamma delta heterodimer which reacted with both anti-pan gamma delta monoclonal antibodies (mAbs) as well as anti- clonotypic mAbs. The availability of this soluble TCR in large amounts should allow us to study the interaction of the gamma delta TCR with its target antigen. In order to further functionally characterize gamma delta T lymphocytes, we have analyzed the requirements for activation of polyclonal populations of gamma deltaT cells isolated from newborn thymocytes. Large numbers of gamma delta T cells can be generated by stimulating CD4-CD8- TCR alpha beta- thymocytes with combinations of cytokines (IL1+2, IL2+7, IL1+7), but not with any of the cytokines individually. Although gamma delta cells failed to proliferate when stimulated with plate-bound anti-pan gamma delta mAb, the addition of certain cytokines (IL1, IL2, and IL7) resulted in a vigorous proliferative responses. These studies demonstrate that cytokine production by gamma delta cells may be less efficient than that of TCR alpha beta+ T cells which can be directly triggered by plate-bound anti-TCR mAbs. Furthermore, it raises the possibility that gamma delta T cells may be passively recruited into sites of inflammation by cytokines produced by alpha beta T cells. We have injected large numbers of gamma delta T cells into SCID mice and observed that following injection of gamma delta cells which bear the V gamma 3 TCR, CD4 and CD8 expression can be induced on precursor cells of SCID origin. These experiments suggest that gamma delta T cells may influence the development of the alpha beta T cell lineage particularly during T cell ontogeny.